Magnetic resonance imaging is commonly used to image internal organs or the interior of a patient. MRI is performed by placing the object to be imaged in a highly uniform and strong (e.g. 1.5 Tesla) magnetic field. The magnetic field causes the atomic nuclei (which possess a magnetic moment) to become aligned with the magnetic field. The nuclei (spins) precess about the magnetic field direction at a rate which is proportional to the magnetic field strength. For hydrogen nuclei (commonly used as the diagnostic species in MRI) in a 1.5 Tesla magnetic field, the precession frequency is about 64 MHz.
In performing imaging, a magnetic field gradient is applied to the object simultaneously with an RF excitation pulse. The magnetic field gradient causes nuclei in different locations to precess at different frequencies (since the precession frequency is dependent upon magnetic field strength). The RF pulse has a well-defined frequency spectrum and polarization, and so causes nuclei having only certain precession frequencies (i.e. in certain locations) to become excited. The process is designed such that a rotating magnetic moment (localized within the volume of interest) is produced. The rotating magnetic moment provides a signal which is easily detectable with a nearby antenna. The entire process of applying gradients and RF pulses is repeated using a sequence of readout gradient encodings to collect a data set that can be reconstructed into an image.
MRI is helpful in guiding interventional devices such as needles into a patient. MRI enables the accurate positioning of the device. Such interventional devices often have a magnetic susceptibility which is different from the magnetic susceptibility of surrounding human tissue. Although usually very small (e.g. about 100 parts per million), this difference in magnetic susceptibility causes problems when performing MRI, which requires a highly uniform field. The interventional device causes the magnetic field to become distorted in magnitude (directional changes are negligible for MRI purposes).
Therefore, when imaging is performed, the resultant image is distorted in the vicinity of the interventional device. Large distortions make it difficult to properly locate an interventional device in a desired location.
One solution to the problem of interventional device imaging has been to make the interventional devices out of a material which has a magnetic susceptibility that closely matches that of human tissue (e.g. carbon fiber). An interventional device with such a magnetic susceptibility will not distort the magnetic field. However, interventional devices made of such materials typically do not work as well in performing their intended function (e.g. acquiring biopsy samples) as devices made of conventional materials.
Another proposed solution to interventional device distortions concentrated on changing the pulse sequence to achieve the best contrast between the device and patient. See Lewin, J. et al. "Needle Localization in MR-Guided Biopsy and Aspiration: Effects of Field Strength, Sequence Design, and Magnetic Field Orientation" in American Journal of Roentgenology, vol. 166, pgs. 1337-1345, 1996, and "Interactive MR-Guided Breast Lesion Localization-A Phantom Study" by B. Daniel in Proc., International Society for Magnetic Resonance in Medicine, New York, 1996, pg. 1733. A problem with these techniques is that the tissue-tissue contrast and the device-tissue contrast are not both optimized in the same pulse sequence.
Total inhomogeneity correction including chemical shifts and susceptibility by view angle tilting, by Z. H. Cho, D. J. Kim, and Y. K. Kim, in Med. Phys. 15(1) January/February 1988, discloses a successful technique for removing the distortions caused by variations in magnetic susceptibility. The method includes the step of applying an additional magnetic field gradient during readout of MR signals to change the angle of MR data readout. This angular tilt corrects for displacements and distortions caused by local variations in magnetic field strength. Reduction of image distortion in the presence of metal, by A. J. McGowan, A. L. Mackay, Q. S. Xiang, D. G. Connell, D. L. Janzen, and P. L. Munk, in the 1997 Proceedings of the International Society of magnetic Resonance in Medicine annual meeting, Vancouver, Canada, Abstract #1973 discloses that the method of view angle tilting can be used to reduce or eliminate distortions caused by metal objects (e.g. a hip joint replacement) in human tissue.
A problem with removing magnetic susceptibility distortions by means of view-angle tilting is that the distortions are often removed completely, rendering the device nearly invisible. This is undesirable because it is often useful to see the device so that it can be guided into its proper location (e.g. when taking a biopsy tissue sample). Imaging parameters such as gradient echo imaging or spin echo imaging may be adjusted to increase the device-tissue contrast so that the device may be seen, but this often precludes the use of parameters which are optimized for tissue-tissue contrast.
Therefore, it would be an advance in the art of magnetic resonance imaging to provide a method of using view angle tilting which allows independent control of device-tissue contrast and tissue-tissue contrast.